The leading edges of aircraft engine nacelles and other aircraft components are prone to ice buildup. FIG. 1 shows a schematic representation of a typical high-speed jet engine assembly 10. Air enters through inlet section 14, between fan blade spinner 16 and an annular housing 18, which constitutes the forward most section of a nacelle 20, and includes nacelle inlet lip 21. Hot, high-pressure propulsion gases pass through the compressor section 17 and the exhaust assembly (not shown) at the rear of the nacelle. An annular space or D-duct 30 is defined by bulkhead 28 and annular housing 18. Bulkhead 28 separates D-duct 30 from the interior portion 31 of the inner barrel 12 of the nacelle. In flight, under certain temperature and humidity conditions, ice may form on the nacelle inlet lip 21, which is the leading edge of the annular housing 18, and on the fan blade spinner 16. Accumulated ice can change the geometry of the inlet area between annular housing 18 and fan blade spinner 16, and can adversely affect the quantity and flow path of intake air. In addition, pieces of ice may periodically break free from the nacelle 20 and/or spinner 16 and enter the engine 50, potentially damaging fan and rotor blades 60 and other internal engine components.
Nacelles also serve an important role in addressing fan noise from the engines, which can be a prime source of overall aircraft noise. As is known to those skilled in the art, aircraft engine fan noise can be suppressed at the engine nacelle inlet 14 with a noise absorbing inner barrel liner 40, which converts acoustic energy into heat. The liner 40 normally includes (as shown in FIG. 6) a face skin 42 having a plurality of spaced openings or perforations 41. The face skin 42 is supported by an open cell core 44 to provide structural support, and to provide a required separation between the porous face sheet 42 and a solid back skin 46. The liner 40 also can include at least one septum 43 that divides each cell into sections, including an upper portion 45 and a lower portion 49. The septum 43 can include a porous membrane or a solid membrane having at least one opening 47 to provide acoustic communication between the upper cell portion 45 and the lower cell portion 49. This arrangement provides effective and widely accepted noise suppression characteristics. Aircraft engines with reduced noise signatures are mandated by government authorities, and often are specified by aircraft manufacturers, airlines and local communities.
U.S. patent application Ser. Nos. 11/276,344 and 11/733,628 (incorporated herein by reference in their entirety), describe graphite fabric heater elements embedded within the layers of a composite structure such as a nacelle inlet lip. The described composite structure includes a heater element integrally formed within a composite aircraft structure having a leading edge. The composite structure includes an open cell core, and a plurality of composite layers atop the core. The composite layers include perforations that extend through the composite layers (including the heater element layer) to the underlying open cell core. The graphite fabric heater elements can include a plurality of interwoven threads containing electrically conductive graphite fibers. Such a structure provides both ice protection and noise attenuation.
As shown in FIG. 2, a typical nacelle inlet lip 100 can be formed in two or more circumferentially extending lip sections 110, 112, 114 that are joined end-to-end. The sections 110, 112, 114 can be connected at spliced joints 115, 117, 119. A detail of one typical prior art spliced joint 119 between the ends of two inlet lip segments 110, 112 is shown in FIG. 3. In this arrangement, the segments 110, 112 meet along a space 120, which usually includes a narrow gap between the opposed ends of the segments 110, 112. One end 126 of a first segment 110 is connected to an adjacent end 122 of a second segment 112 by a plurality of fasteners 121 that extend through the segments 110, 112 and connect to a backing plate or splice plate (not shown in FIG. 3) that spans rear portions of the adjoined ends 122, 126 of the segments 110, 112 and the gap 120 therebetween, thus securing the ends 122, 126 in end-to-end relationship. As shown by dashed lines in FIG. 3, when the leading edges of the segments 110, 112 are provided with integral electrically powered ice protection heaters 123, 127, the ends 125, 129 of the heaters nearest the gap 120 are spaced apart by a circumferential distance W1. The spacing W1 is necessary in order that the metal fasteners 121 do not extend through or contact the electric heater elements 123, 127.
Another typical prior art spliced joint 419 between two adjoined inlet lip segments 410, 412 is shown in FIGS. 4A and 4B. In this arrangement, a splice plate 424 is positioned on adjacent exterior faces of the segment ends 422, 426, and across a gap 420 therebetween. A plurality of fasteners 421 extend through the splice plate 424 and the segment ends 422, 426, thus securing the ends 422, 426 together in end-to-end relationship. As shown in FIGS. 4A and 4B, integral electric heater elements 423, 427 can include a plurality of spaced electrically conductive bus strips 430 for use in establishing an electric potential across the heater elements 423, 427. As also shown in FIGS. 4A and 4B, the bus strips 430 can be connected to a voltage source by wires 433 that extend through the back sides of the inlet lip segments 410, 412. Like the back-splice arrangement discussed above, the edges of heater elements 423, 427 that are nearest the gap 420 are necessarily spaced apart by a circumferential distance W2 such that none of the metal fasteners 421 penetrate the heater elements 423, 427 or the bus strips 430.
In some circumstances, the exterior surfaces of the inlet lip segments 110, 112 associated with the gaps W1 and W2 shown in FIGS. 3-4B may not be sufficiently heated by the nearest heater elements 123, 127 to prevent ice formation or to melt accumulated ice. Accordingly, these unheated gaps W1, W2 can result in “cold spots” at the gaps 120 between adjoined inlet lip segments. As discussed above, ice accumulation on the surfaces of an aircraft's leading edges is undesirable, particularly on the leading edge of an aircraft engine nacelle. In addition, the spliced joints described above can substantially prevent effective acoustic treatment of the portions of the inlet lip segments associated with the gaps W1, W2 because connecting hardware such splice plates, mechanical fasteners, and the like, can at least partially block acoustic perforations in the outer skin and/or the cells of an underlying cellular core, or otherwise interfere with optimal performance of the acoustic liner 40.
Accordingly, there is a need for an ice protection system for an aircraft component's leading edge that includes heating elements that cover substantially the entire extent of the aircraft component's leading edge surface, including those portions of the component that are immediately adjacent to a structural joint between adjacent component segments. In addition, there is a need for such a system that also includes acoustic treatment of substantially the entire extent of the aircraft component's leading edge surface, including those portions of the component that are immediately adjacent to a structural joint between adjacent component segments.